Chronic arsenic (As) exposure currently affects roughly 140 million people worldwide. Methylation of ingested inorganic arsenic (InAs) to methylarsonic- (MMA) and dimethylarsinic acids (DMA) relies on folate- dependent one carbon metabolism and facilitates urinary arsenic (As) elimination. We recently completed a randomized placebo-controlled trial of folic acid (FA) supplementation in 200 folate-deficient As-exposed residents of Araihazar, Bangladesh. The results indicate that folate deficiency and hyperhomocysteinemia (HHcys) are associated with a reduced capacity to methylate As and are risk factors for As-induced skin lesions. Furthermore, FA supplementation facilitates As elimination and significantly lowers blood As concentrations in individuals who are folate deficient. We have also determined that blood As is a good biomarker of As exposure and is directly associated with the risk for As-induced skin lesions. While these findings are extremely compelling, several fundamental questions must be addressed prior to a large scale intervention aimed at assessing the possible impact of FA supplementation on As-induced disease-related outcomes. The proposed studies will address the following questions: Aim 1: a) Does FA supplementation lower blood arsenic concentrations in the general population? b) Does a higher dose of FA lead to an incremental lowering of bAs?, Aim 2: a) What is the time-course of the decline in blood As, and at what point is a nadir in blood As achieved? b) Is there a rebound in blood As after the cessation of FA supplementation due to release of As from tissue stores? Aim 3: a) Can the ability of FA to lower blood As and homocysteine be enhanced by the addition of a novel new alternative approach: reduce methylation demand. We have previously found that urinary creatinine (a catabolite of creatine) is by far the strongest predictor of As methylation, and that participants with lower urinary creatinine are at increased risk for As- induced skin lesions. Urinary creatinine is influenced by dietary intake of creatine (derived from meat), which downregulates endogenous creatine biosynthesis. Since creatine biosynthesis is the major consumer of methyl groups, we will test the hypothesis that creatine supplementation will spare methyl groups, lower homocysteine and facilitate the methylation of As, and thereby lower blood As. To answer all of these questions, we propose to conduct a randomized, double-blind, placebo controlled trial of FA (a comparison of two doses and durations), creatine, and creatine plus FA. Positive results of these interventions would have enormous therapeutic potential for ameliorating the long-term health consequences of As exposure for the many populations at risk.